Nowadays electric power energy systems are critically important social infrastructure which cannot be stopped even for a moment. However, in an abnormality or a trouble of a load that causes an overcurrent, a measure taken thereagainst is a high-speed breaking of the load, as is exercised by a fuse or a high-speed mechanical switch. Nevertheless, there has been a demand for a highly functional switch, so called a controller or a current limiter, which is capable of limiting only the overcurrent and allowing a continued operation without the complete stop of the load, as well as a system recovery to a full operation after the return to its normality.
Electric power system must be designed to withstand a short-time overcurrent that can be caused due to such as a rush current of an incandescent lamp being lit, a start-up rush of an induction motor or an initial excitation inrush of a transformer. It is important to distribute yield capacity of each machine appropriately. A semiconductor-type inverter power supply in recent years, such as a fuel cell inverter, for example, cannot withstand, in many cases, a peak current which is almost ten times the excitation inrush current of a transformer. Inverter power supplies, therefore, have various soft-start functions, which work if there is one load for one inverter power supply but may not work for late-started ones of a plurality of loads connected to one inverter power supply.
Electric power systems are designed in consideration of protective coordination, the current and the duration thereof to withstand an accidental, short-time overcurrent. However, such systems merely perform a protective coordination aimed at a prevention of the influence over the upstream by selectively breaking the accident current by a switch. It is a recent social demand to achieve a continuous operation as far as possible without power breaking if the accident has taken place in the downstream of a system.
As for a current limiter which limits an accident current with series elements, an application based on a transient phenomenon between superconductive mode and normal conductive mode is developed. This is for size and cost reduction of the breaker, whose required capacity is extremely large when the accident current is excessively large: such size and cost reduction can be attained if only the accident current is reduced into the half or so by the current limiter.
In the case of a three-phase transformer, an exciting rush current due to iron core saturation is expected. Therefore, it is necessary that such transformers be constructed to possess an overcurrent withstand capacity against electromagnetic force of the electric wire.